It is a well established practice in the automotive industry to apply an acidified zinc phosphate solution to metal car bodies to form a zinc phosphate coating on the metal surfaces for corrosion protection. The kernel of this process is a chemical reaction between the substrate and acid replenisher containing zinc and metal cations, and phosphate and fluoride anions. The rate of the reaction is determined by an accelerating agent--nitrite concentration. The coating quality and corrosion performance are strongly related to the precise control of the phosphate solution constituents within specified narrow ranges. Optimal phosphate coatings are obtained only if the major bath constituents are maintained within narrow regions. A phosphate bath is characterized with dead times of about 20-30 minutes. These time constants are dependent on the bath constituents and vary over time. Overshooting is to be avoided because it takes a long time for the bath to recover from an excess of some component. Furthermore, overshooting produces a considerable amount of sludge.
One prior art phosphate bath control system approach uses programmable logic controllers (PLC) to control constituents. These systems monitor a conductivity probe which provides information about the integral metal cations concentration in the bath. Chemical feed control for metals and total acid are based on total bath concentration rather than on individual parameter measurements. Titrations for total acid, nitrite, and free acid are performed manually. Metal concentrations are not measured in the plant but analyzed every 2-3 weeks and therefore critical information is not available until long after the event. FIG. 1 shows a plot of the Zn concentrations of a phosphate bath under PLC control based on conductivity measurements. The result of this type of control are significant fluctuations of the metal components which are associated with unstable quality of coating and production of considerable amounts of sludge. Control systems that are based on automatic titrations of metals, total acid, free acid and nitrite are also available. However, these systems are of low reliability in a production environment and require sophisticated maintenance.
One straightforward alternative is a (linear) PID control strategy, which is a well established control strategy that does not require special efforts for realization, implementation, and tuning. However, there are several major concerns regarding implementation of a conventional linear PID control algorithm for phosphate bath control. These concerns are:
nonlilnearilty of system dynamics; PA1 significant and variable dead times; PA1 wide operating range--regulation for small and large (start-up phase) deviations from the setpoint; PA1 heuristic information cannot be formalized analytically.
Simulation studies show the lack of robustness of the PID control of the Zn concentration for significant changes in process conditions. Control performance drastically decreases for a large perturbation of the Zn gain of the replenisher. Another difficulty related to the PID control is the problem of including the heuristics in the control algorithm. This task could contribute significantly to the complexity of the control strategy.